As httpx strives for close compatibility with requests few
changes needed to be made in curlify:
* .lower() on headers (headers in http are case insensitive
  and httpx / requests use different cases.
* httpx does not have a .body but contents can be accessed
  via .read() instead, curlify will now use either or.
* httpx stores url in as URL object so we stringify it.

Majority of the changes are in tests.
* all tests are mocked out so there is no hitting of external
  services
* payload tests are more relaxed. Testing for approximate
  existence of flags rather than exact string match. This is a
  workaround to deal with various discordant optional http
  headers.